In a chain drive, when a roller chain is in mesh with a sprocket to transmit power, wear is produced on the tooth surfaces of the sprocket, particularly as a result of sliding contact at the location where the sprocket meshes with the chain, as described in detail below.
Referring to FIG. 5 of the drawings, when a sprocket tooth 11 and chain roller 12 mesh with each other, contact usually occurs at a fixed contact position 13. At the contact position, a force F is applied by the roller 12 to the sprocket tooth surface 11a in a fixed direction. The force F acts along a direction 14, which corresponding to the pressure angle θ determined by the shape of the sprocket teeth.
The force F results in increased contact surface pressure at the contact position, and generates wear on the tooth 11 as a result of sliding contact both during meshing engagement of the roller with the sprocket tooth and during disengagement of the roller from the sprocket tooth. The wear of the sprocket tooth 11 progresses along the line of action, i.e., along the direction 14 corresponding to the pressure angle θ.
In a roller chain transmission, the pressure angle θ is usually defined as the angle formed between the chain pitch line 15 and a line 14 extending from the center of a roller 12 and perpendicular to the contact surface of the sprocket tooth, as shown in FIG. 6. The chain pitch line 15 intersects the pitch circle 15a of the sprocket at the locations of the centers of two successive rollers, as if the centers of both of the two successive rollers were on the pitch circle.
There is a limit to the amount of wear that a sprocket tooth can sustain while still being able to mesh smoothly with a roller chain. When the wear of the sprocket teeth exceeds this limit, the meshing of the sprocket and the chain is no longer smooth, and tooth defects are generated in the sprocket. Thus, it has been necessary to determine the wear limit of the sprocket.
To detect the extent of wear in a sprocket tooth, several methods have been used in the past. As shown in FIG. 7, a tooth form gauge D has been used. The gauge is brought into contact with the sprocket, and the gap between the tooth surface and the gauge is measured. Alternatively, the shape of the profile of a sprocket tooth is transferred onto paper by ink rubbing, so that the difference between the original tooth form and the current tooth form can be measured. Another method, as shown in FIG. 8, is to provide a concave groove 11b on each tooth surface 11a of the sprocket, so that the disappearance of the groove due to wear of the tooth surface can be observed visually (see Japanese Laid-Open Utility Model Publication No. Hei. 3-78153).
The use of a gauge for gap measurement, as shown in FIG. 7, is subject to several problems. Gap measurement requires visual observations, which may be made differently by different individuals, and may also be dependent on the particular measurement technology which is used. The sprocket is not always readily accessible. If it is hidden behind other machine components, tooth form gauge measurement may not be possible. Furthermore, it is always necessary to keep a tooth form gauge available for each different sprocket.
The ink-rubbing transfer method is subject to errors due to shift and elongation of the paper. Curvature of the tooth forms also prevents their shapes from being correctly transferred onto paper. Moreover, it is necessary to maintain a reference tooth form for measurement and determination of wear loss.
The use of a concave, wear-indicating groove is also subject to a number of problems. Since the groove is positioned on each chain-contacting tooth surfaces, it reduces the available contact area, thereby increasing the contact surface pressure and increasing the rate at which wear of the sprocket teeth progresses. Formation of the concave groove on the tooth surfaces is difficult and costly. A mixture of fats and oils and powder resulting from wear of the sprocket teeth and other components such as conveyor components, tends to accumulate in the groove, and must be removed for proper observation or measurement of the wear of the sprocket teeth. Finally, as in the case of the tooth form gauge, if the sprocket is hidden behind other machine components, the concave groove is difficult to see, and consequently the amount of wear is difficult to determine.
Accordingly, an object of the invention is to solve the above-mentioned prior art problems. More particularly, an object of the invention is to provide a sprocket with a wear limit indicator, which makes it possible to determine easily and reliably whether or not the wear conditions of the sprocket teeth are within the acceptable limit. It is also an object of the invention to make it possible to observe tooth wear conditions from a side of the sprocket, and to facilitate observation of wear in those cases where the sprocket is installed in a machine at a location at which it not readily accessible.